We determined the contribution of calcium-independent phospholipase A2β (iPLA2β) to lung metastasis development following breast cancer injection into wild-type (WT) and iPLA2β-knockout (iPLA2β-KO) mice. WT and iPLA2β-KO mice were injected in the mammary pad with 200,000 E0771 breast cancer cells. There was no difference in primary tumor size between WT and iPLA2β-KO mice at 27 days postinjection. However, we observed an 11-fold greater number of breast cancer cells in the lungs of WT mice compared with iPLA2β-KO animals ( P < 0.05). Isolated WT lung endothelial cells demonstrated a significant increase in platelet-activating factor (PAF) production when stimulated with thrombin [1 IU/ml, 10 min, 4,330 ± 555 vs. 15,227 ± 1,043 disintegrations per minute (dpm), P < 0.01] or TNF-α (10 ng/ml, 2 h, 16,532 ± 538 dpm, P < 0.01). Adherence of E0771 cells to WT endothelial cells was increased by thrombin (4.8 ± 0.3% vs. 70.9 ± 6.3, P < 0.01) or TNF-α (60.5 ± 4.3, P < 0.01). These responses were blocked by pretreatment with the iPLA2β-selective inhibitor ( S)-bromoenol lactone and absent in lung endothelial cells from iPLA2β-KO mice. These data indicate that endothelial cell iPLA2β is responsible for PAF production and adherence of E0771 cells and may play a role in cancer cell migration to distal locations.